You probably would’ve noticed the dotted underlined red lines under misspelt words which are typed into text boxes/forms/fields. To me, it’s a rather welcomed feature since I use web applications and services quite a fair bit (I don’t misspell a lot, but automated checking is a bonus I guess ;)), but by default, the bundled dictionary is for the US locale which would complain when you spell ‘favour‘ instead of ‘favor‘.
Fortunately, it’s a rather easy to get around this, which is just to select and install the appropriate dictionary from here. Once that’s done, just right-click on any text box/field/form and you should be able to select the dictionary you want to use (screenshot below):
Once that’s done, you’re all set! :)